Adjust oven rack to the upper position and pre-heat the broiler to high.

Line a rimmed baking sheet with aluminum foil. Place the tomatillos and jalapeno cut side down on the baking sheet. Add the onion and garlic clove to the baking sheet and spread all ingredients evenly. Broil for about 5-7 minutes, or until most ingredients are blistered and blackened. You may need to remove the blackened items and return the rest to cook under the broiler for a few more minutes. Once cool enough to handle, peel the garlic clove.

Transfer all ingredients to a blender and blend on high until ingredients are fully broken down and incorporated, about 15 seconds. Taste and adjust seasoning if needed.